Incremental scenarios describe techniques where particular climatic (or related)
elements are changed incrementally by plausible though arbitrary amounts (e.g.,
+1, +2, +3, +4°C change in temperature). Also referred to as synthetic scenarios
(IPCC, 1994), they are commonly applied to study the sensitivity of an exposure
unit to a wide range of variations in climate, often according to a qualitative
interpretation of projections of future regional climate from climate model
simulations (“guided sensitivity analysis”, see IPCC-TGCIA, 1999).
Incremental scenarios facilitate the construction of response surfaces –
graphical devices for plotting changes in climate against some measure of impact
(for example see Figure 13.9b) which can assist
in identifying critical thresholds or discontinuities of response to a changing
climate. Other types of scenarios (e.g., based on model outputs) can be superimposed
on a response surface and the significance of their impacts readily evaluated
(e.g., Fowler, 1999). Most studies have adopted incremental scenarios of constant
changes throughout the year (e.g., Terjung et al., 1984; Rosenzweig et al.,
1996), but some have introduced seasonal and spatial variations in the changes
(e.g., Whetton et al., 1993; Rosenthal et al., 1995) and others have examined
arbitrary changes in interannual, within-month and diurnal variability as well
as changes in the mean (e.g., Williams et al., 1988; Mearns et al., 1992; Semenov
and Porter, 1995; Mearns et al., 1996).

Incremental scenarios provide information on an ordered range of climate changes
and can readily be applied in a consistent and replicable way in different studies
and regions, allowing for direct intercomparison of results. However, such scenarios
do not necessarily present a realistic set of changes that are physically plausible.
They are usually adopted for exploring system sensitivity prior to the application
of more credible, model-based scenarios (Rosenzweig and Iglesias, 1994; Smith
and Hulme, 1998).

13.2.2 Analogue Scenarios

Analogue scenarios are constructed by identifying recorded climate regimes
which may resemble the future climate in a given region. Both spatial and temporal
analogues have been used in constructing climate scenarios.

13.2.2.1 Spatial analogues

Spatial analogues are regions which today have a climate analogous to that
anticipated in the study region in the future. For example, to project future
grass growth, Bergthórsson et al. (1988) used northern Britain as a spatial
analogue for the potential future climate over Iceland. Similarly, Kalkstein
and Greene (1997) used Atlanta as a spatial analogue of New York in a heat/mortality
study for the future. Spatial analogues have also been exploited along altitudinal
gradients to project vegetation composition, snow conditions for skiing, and
avalanche risk (e.g., Beniston and Price, 1992; Holten and Carey, 1992; Gyalistras
et al., 1997). However, the approach is severely restricted by the frequent
lack of correspondence between other important features (both climatic and non-climatic)
of a study region and its spatial analogue (Arnell et al., 1990). Thus, spatial
analogues are seldom applied as scenarios, per se. Rather, they are valuable
for validating the extrapolation of impact models by providing information on
the response of systems to climatic conditions falling outside the range currently
experienced at a study location.